Project Summary Vision is useful because it informs us about physical properties of the environment. In the case of color, one important function is to provide information about object identity and physical properties - color is generally considered a perceptual correlate of object surface reflectance. For color to be a reliable in this regard, the perceived color of an object must remain stable across variations in the scenes in which it is viewed. This requirement is non-trivial because the light reflected to the eye varies with scene factors that are extrinsic to the object, such as the spectrum of the illumination and the reflectance of nearby objects. The visual system adjusts to stabilize (partially) object color appearance against changes in these extrinsic factors, a phenomenon called color constancy. This proposal consists of experiments designed to study object color appearance and its constancy. Color constancy is an example of a larger class of perceptual constancies (e.g. size constancy and shape constancy) that together allow us to perceive a stable physical world. As such, characterization of the color system may provide insights that generalize beyond color per se. The proposed research consists of behavioral experiments with human observers that will allow a functional characterization of how we perceive object color, as well as other perceptual correlates of object surface reflectance. Questions to be addressed include a) how does the visual system integrate information across the surface of three-dimensional objects to arrive at an overall percept of object color? b) how stable is object color appearance across variation in object shape and pose? c) how do changes in object spectral reflectance interact with changes in object material properties (i.e. changes in the object's bidirectional reflectance distribution function) in the perception of object surface properties? and d) can we generalize theories of color constancy developed for flat matte objects seen under spatially diffuse illumination to the case of three-dimensional objects viewed in geometrically rich three-dimensional scenes? This research should help us understand how color vision works in the real world and provide a foundation for clarifying how the brain provides functionally useful representations of objects.